# Finitely presentable objects in functor categories

Given a locally finitely presentable category $\mathcal{C}$ it is well-known that every functor category $[\mathcal{A},\mathcal{C}]$ (where $\mathcal{A}$ is a small category) is also locally finitely presentable. Is there a concrete description of the finitely presentable objects in such categories?

If this is not possible in general, what about special cases like $\mathcal{A}=\mathcal{Set}_{fin}^{op}$?

• Coequalizers of finite coproducts of representables. – Fernando Muro Jan 20 '14 at 21:53
• @FernandoMuro What representables? $\mathcal{C}$ is not $\mathbf{Set}$. – Todd Trimble Jan 21 '14 at 2:52
• Well, your conditions ensure that C is equivalent to [B, Set] for B the category of finitely presentables in C, so you end up with [AxB, Set] – Fernando Muro Jan 21 '14 at 8:01
• Sorry, preserving finite limits in B. – Fernando Muro Jan 21 '14 at 8:33

When $\mathcal{A}$ is a finite category, the finitely presentable objects in $[\mathcal{A}, \mathcal{C}]$ are precisely the diagrams that are componentwise finitely presentable: see e.g. Proposition 2.23 here. The general case is harder to describe. There are two steps:
1. First, determine the finitely presentable objects in $[\operatorname{ob} \mathcal{A}, \mathcal{C}]$; these will contain e.g. "finitely supported" families of finitely presentable objects.
2. Then, determine the left adjoint of the restriction functor $[\mathcal{A}, \mathcal{C}] \to [\operatorname{ob} \mathcal{A}, \mathcal{C}]$; by general nonsense, the restriction functor is finitely accessible and monadic, and the finitely presentable objects of $[\mathcal{A}, \mathcal{C}]$ will be the objects in the smallest full subcategory that is closed under finite colimits and that contains the free "algebras" generated by the finitely presentable objects in $[\operatorname{ob} \mathcal{A}, \mathcal{C}]$.